Automatically generate keys for module signing if they're absent so thatallyesconfig doesn't break. The builder should consider generating their ownkeyrings, however, so that the keys are appropriately named and any extra keysrequired get imported.

Also change the names of the keyring files to modsign.pub and modsign.sec sothat they are then a more obvious what they're about and add a dependency forthe signing rules on the keyring files so that the signatures get regeneratedif the keyrings change.